Fix improper NULL handling in list partitioning code.

The previous logic was wrong when the value was NULL but there was
no partition for NULL.

Amit Langote, reviewed by Jeevan Ladhe

Discussion: http://postgr.es/m/d64f8498-70eb-3c88-b56d-c54fd3b0500f@lab.ntt.co.jp

src/test/regress/sql/insert.sql

@@ -226,6 +226,12 @@ insert into mlparted values (1, 2);
 -- selected by tuple-routing
 insert into mlparted1 (a, b) values (2, 3);
 
+-- check routing error through a list partitioned table when the key is null
+create table lparted_nonullpart (a int, b char) partition by list (b);
+create table lparted_nonullpart_a partition of lparted_nonullpart for values in ('a');
+insert into lparted_nonullpart values (1);
+drop table lparted_nonullpart;
+
 -- check that RETURNING works correctly with tuple-routing
 alter table mlparted drop constraint check_b;
 create table mlparted12 partition of mlparted1 for values from (5) to (10);